Delta Air Lines Inc. must pay a $2 million civil penalty, following a Department of Transportation (DOT) investigation released Thursday that said the airline violated rules protecting passengers with disabilities.

Delta agreed to settle the case—the DOT’s largest fine against an airline in a non-safety-related case—without admitting wrongdoing.

Among the agency’s complaints were that Delta allegedly failed to properly assist passengers with disabilities on and off planes; respond adequately to written complaints; and report each complaint to the DOT.